The invention relates to a system and method for controlling an internal combustion engine coupled to an emission control device.
In direct injection spark ignition engines, the engine operates at or near wide-open throttle during stratified air-fuel ratio operation in which the combustion chambers contain stratified layers of different air-fuel ratio mixtures. Strata closest to the spark plug contain a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, a three way catalyst optimized for NOx storate, known as a NOx trap or catalyst, is typically coupled downstream of the first three-way catalytic converter.
During lean, rich, and stoichiometric operation, sulfur contained in the fuel can become trapped in the emission control device in the form of SOx. This gradually degrades emission control device capacity for storing NOx, as well as emission control device efficiency. To counteract sulfur effects, various sulfur decontamination methods are available.
One method for determining degradation of a NOx trap due to sulfur contamination uses an estimate of NOx trap capacity, or a NOx absorption amount. In particular, NOx trap capacity is estimated by completely saturating the trap with NOx and then calculating an amount of NOx stored from fuel used to purge stored NOx. Such a method is disclosed in EP 732 250.
The inventors herein have recognized numerous disadvantages if trap capacity is used to determine when to perform a decontamination cycle. In particular, such an approach may request decontamination cycles too often, thereby degrading fuel economy since there is a fuel penalty for performing decontamination cycles. In other words, known methods for performing decontamination cycles require a certain fuel amount to generate heat and raise temperature. This fuel amount does not contribute to motive force and thereby can degrade vehicle fuel economy.
Similarly, such an approach may request decontamination cycles too infrequently. Such operation also degrades fuel economy since more frequent fill and purge cycles will be necessary to meet regulated emissions. In other words, contamination results is lower capacity and less fuel efficient NOx purging, which can decreases overall fuel economy.
Another method for determining when to perform decontamination cycles uses an estimation scheme. In this approach, an amount of SOx stored in the NOx trap is estimated based on operating conditions. For example, an amount of SOx stored is estimated based on driving distance. In another example, the amount of stored SOx is estimated based on engine operating conditions. Then, when the estimated amount of stored SOx reaches a predetermined value, the decontamination cycle is performed. Such a method is described in U.S. Pat. No. 5,657,625.
The inventors herein have recognized a disadvantage with the above approach. In particular, depending on the setting of the predetermined value, fuel economy will be significantly affected. For example, if the value is set too high, decontamination cycles will be too frequent. With too frequent decontamination, fuel economy will be degraded since fuel is too often spent to perform decontamination. Similarly, with too infrequent decontamination, fuel economy may be degraded since fill and purge cycles will be inefficient.
An object of the invention claimed herein is to provide a method for enabling emission control device decontamination cycles.
The above object is achieved, and disadvantages of prior approaches overcome, by a method for controlling an engine coupled to an emission control device susceptible to reversible contamination, the engine capable of operating at a first operating condition and a second operating condition, the method comprising determining an impact of operating the engine at the first operating condition compared to operating the engine at the second operating condition, and performing a decontamination cycle in response to at least said impact, wherein said decontamination cycle reduces the reversible contamination.
By enabling emission control device decontamination cycles based on impacts of different operating conditions, it is possible to determine whether performing decontamination cycles will improve, degrade, or have minimal impact on vehicle performance. In other words, it may be the case that performing decontamination will allow longer lean operation and therefore improved fuel economy. However, it may also be the case that the act of performing decontamination will degrade fuel economy greater than any gains by longer lean operation, thereby giving degraded fuel economy. The present invention allows improved operation to overcome these obstacles.
An advantage of the above aspect of the present invention is improved fuel economy without degrading emission performance.
In another aspect of the present invention, the above object is achieved and disadvantages of prior approaches overcome by a method for controlling an engine coupled to an emission control device, the engine capable of operating at a first operating condition and a second operating condition, the method comprising determining a fuel savings of operating the engine at the first operating condition; determining a fuel loss of operating the engine at the second operating condition; and enabling one of the first operating condition and the second operating condition based on said fuel savings and said fuel loss.
By considering both a fuel loss and a fuel savings, it is possible to determine whether to enable operating conditions so that optimal fuel economy is obtained. For example, when lean operation is the first operating condition and a decontamination cycle is the second operating condition, if the fuel loss is greater than the potential fuel savings, then a decontamination cycle may not be justified.
An advantage of the above aspect of the present invention is improved fuel economy without degrading emission performance.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.